The instrument height problem

The Trimble S6 has a height notch, and when you enter it into the setup screen it converts to true vertical.

I feel this gives an HI accuracy of 0.001 m.

It is not coincident with the horizontal axis, so you need to apply the radius to the notch and the vertical distance up to the axis. But then your solution needs a mathematical reduction as well, just not the vertical offset.

In terms of the radius of the average total station (100mm), set up at an average height (1500mm, I've found that solving the triangle for the difference between slant vs vertical height {225000 - 10000 = 224000, sqrt of 224000 = 1497) is about 3mm.

> Thought I'd try a rough prototype of something that might make IH measurement easier. Based on the GPS 'slant height' method, this would require a conversion from slant to vertical but that could be covered by a look up table carried in the instrument box.

Have you experimented with measuring the HI to a lower edge of the instrument body (above the tribrach) or to a feature such as a screw on it? If you're using a lookup table, the reduction shouldn't be a problem. You can use just an ordinary tape with a wide enough blade that it is semi-rigid over distances of about 1.6m.

Good point Kent. I think I've been approaching this with a view to solving two problems and the major one is due to working in a windy climate in exposed places (35mph gusting to 44 today!) no tape is rigid enough. But being able to physically hook it at the top and hold it with one hand near the tripod head and pull the point down on the station and lock (GPS style) is a lot easier than trying to hold the zero end on the station with your boot and reading against a cross on the inst.

I use a folding rule, graduated in meters (millimeters) and ft (hundredths). These are made of wood and are pretty rigid. I also use a pocket tape graduated in millimeters, but the folding rule is better in the wind.

On that topic, Trimble has by far the best system, as explained above. Simple and accurate.

Are we building tiny watches or what?

All this talk and then your pole tip sinks more than that before the system is satisfied with a result and records such as being more accurate than measuring a gnat's patootie.

Are we building tiny watches or what?

Actually, in the structural construction world, having an accurate H value at the total station is very much of a standard. I use remote benchmark myself when the elevation is important*. The software creates a point with a new H value; a good check on the taped up value too.

*That does not take away the use of a level, but there are shots where a leveling rod does not fit in, whereas a mini prism do.

Are we building tiny watches or what?

Have you checked to make sure the tape is standardized........

Are we building tiny watches or what?

The slant HI vs. the vertical HI is only different by about 0.01'. This won't make a huge difference in the boundary world, but once you get into construction, aeronautical, shipbuilding, etc., 0.01' is a big deal.

> I use a folding rule, graduated in meters (millimeters) and ft (hundredths). These are made of wood and are pretty rigid. I also use a pocket tape graduated in millimeters, but the folding rule is better in the wind.

I just put my boot on the hook end to hold it on the ground mark. In very windy conditions (when I'm usually not trying to survey, anyway) using a lath to both hold the hook on the ground mark and give it some lateral support should work perfectly well.

Another thing to consider

Is everyone making absolutely certain that their instrument is not sinking over time. A prime example of that was occuring this week with a standard tripod setup on a county road. It had rained, then frozen. As the morning progressed and the temps rose significantly above 32F the road surface began to soften. It didn't matter how hard one attempted to set in the tripod tips beforehand.

I understand that those doing high quality construction-type work daily know how to ensure the best possible results. I am suggesting, however, that some are being meticulous in one way while ignoring the reality around them, thus assuming they are producing better answers than what they truly are achieving.

Another thing to consider

> Is everyone making absolutely certain that their instrument is not sinking over time. A prime example of that was occuring this week with a standard tripod setup on a county road. It had rained, then frozen. As the morning progressed and the temps rose significantly above 32F the road surface began to soften. It didn't matter how hard one attempted to set in the tripod tips beforehand.
>
> I understand that those doing high quality construction-type work daily know how to ensure the best possible results. I am suggesting, however, that some are being meticulous in one way while ignoring the reality around them, thus assuming they are producing better answers than what they truly are achieving.

Yep need to check at least 1 backsight for angle distance and elevation to confirm "accurate" (hopefully) work.
If conditions look changeable then regular checking will save redoing the lot when you discover your tripod has sunk/twisted whatever..

I am frequently checking "yesterdays" work ie doing the same measurements from different positions and am always a bit disappointed with the actual repeatable accuracy compared to what you hope for (the manufacturers spec).
There are certainly plenty of folk that carry on like they're actually measuring everything to a mm without doing much verification.

Another thing to consider

In the winter months it is best to set your tripod on three strategically placed hubs. Even they could settle but not as much as a sun heated tripod foot.

Once upon a time, I recall seeing a WILD Heerbrugg setup with a four-legged tripod that seemed to work just like the old KERNs. It used the fourth leg for tribrach forced centering (some kind of way) similar to the spherical head on a KERN tripod with the fourth leg. I never understood how the 'WILD" new-fangled system could work without a spherical head.

Maybe it didn't ... maybe that's why it never caught on.

The KERN system was phenomenal for speed and automatically gave you your HI.

I've got a couple of them for my DKM-3A; one adjustable legs, one stiff leg.

I think that's the slickest way to get your HI.

The vertical is usually .01 less than the slope with a 5'+/- HI. Deduct .01 form your slope distance and you should be good for most applications. If you need better accuracy, then you you probably should be using equipment capable of first order accuracy. If you need accuracy to the .001', then your typical 6" total station isn't going to cut it.

Here's another approach, not really suitable for day-to-day work, but one that I'm going to use on some river crossings for an upcoming Corps CEPD project. I made two of them, just sections of aluminum channel (scavenged from an old track lighting system) with a 5/8"x11 nut in the middle. We'll record the slant height at the bottom of each end of the crossbar in all 3 positions around the tripod, average them out, correct for slant (crossbar length is known), then subtract the height of the puck to get the height of the tribrach contact pads. Add the height of the instrument above the pads (and reality check with a tape to the instrument) to derive HI.

> In terms of the radius of the average total station (100mm), set up at an average height (1500mm, I've found that solving the triangle for the difference between slant vs vertical height {225000 - 10000 = 224000, sqrt of 224000 = 1497) is about 3mm.

i endorse this post:good:

it's near enough -3mm correction for a 0.1 radius total station at normal setup heights. read tape > subtract 3mm > survey.

Another thing to consider

Shoot, even I can remember the days when we were running an SMI card in a 48, and we would actually "turn elevations off" ! 🙂
Sometimes I enjoy being a boundary surveyor in a colonial state. I do like the comment about using three hubs to set the tripod on, but that'd be a heck of a lot of pounding in this particular winters' frost.